The recycling of used lubricating oils is a problem of increasing economic and ecological importance. The regeneration carried out even today on a large scale by the so-called acid tar process (sulfuric acid extraction and bleaching earth [fuller's earth] hot contact distillation) is unsatisfactory because of the associated waste problems (acid tar, bleaching earth filter cakes), technological problems (corrosion), and the poor yield. Another process has always been constantly sought which leads economically to high yields of high value second raffinates and is also environmentally favorable and produces as little waste as possible.
Particular attention was here directed to processes which operate completely without acid treatment. Mostly these are concerned with a succession of distillation, hydrogenation and filtration. Catalytic hydrogenation has not, of course, been able to be used because of the poisoning of the catalysts and on cost grounds.
Two further processes, described in German Patentschriften Nos. 1,105,543 and 2,508,713, carry out a chemical treatment with sodium metal, but however only partially alleviate the disadvantages mentioned above.
Thus it is still required, when the process according to DE-PS 1,105,543 is carried out, to process the used oil treated with sodium metal with a high added quantity of filter aids (bleaching earth), leading to high amounts of waste and considerable losses of yield. The process described in DE-PS 2,508,713 proposes processing of used mineral oil by purifying the used oil by coagulation and successively by dehalogenating with sodium metal, fractionally distilling, and hydrogenating. The high number of process steps already militates against economically carrying out this process, since losses of yield and waste problems are associated with each process step.
It is also known that the treatment of used oil with chemical reagents is per se undesirable, since these reagents are in general used up and finally appear in some form of waste.
It has thus also been proposed to process a used lubricating oil without chemical aids but only with distillative measures. It is proposed in German Offenlegungsschrift No. 2,628,763 to pre-distill a used oil for at least 4 hours below its cracking temperature, to dry it and to remove the light oil components, and to subject the predistilled material, below its cracking temperature, to a vacuum distillation under very low pressure. This distillation is to be carried out at a pressure of 0.1-2 mm of mercury in a molecular distillation apparatus in a temperature range of 249.degree.-345.degree. C. A yield of 91% is obtained. In practice it is of course found that only very few used oils, specially collected and with little contamination, can be processed by this process. Re-fining of the considerably more strongly contaminated used oils encountered in practice requires, on the other hand, a further treatment, also described in DE-OS 2,628,763, by the acid tar--bleaching earth--hot contact distillation process. Considerable losses of yield thus naturally result, and considerable quantities of wastes arise.
A certain amount of progress is obtained with the recently proposed "Recyclon-Process".RTM., in which, after a dewatering and gasoline removal stage, a treatment is carried out with small amounts of finely divided sodium. For separation of the re-refined lubricating oil, two distillations are then carried out in a molecular distillation apparatus under gentle conditions. According to this process, a wide spectrum of used oils arising in practice can give high value raffinates.
A large part of the disadvantages attributed to the other processes can be eliminated. However, there still arises 18% of a highly alkaline, tarry residue in this process; the yield of usable re-raffinates if only about 77%.
A further improvement is obtained by modifications of the "Recyclon-Process".RTM., which use the teachings of DE-OS 2,813,200. Here process water is added to the used oil after the sodium treatment. But 16% of residues still arise even in this process; the yield of usable re-raffinates is 83%, based on dried, gasoline-removed oil.
A used oil regeneration process was therefore sought which has a low chemicals requirement, produces little residue, can process a wide spectrum of used oils arising in practice, and simultaneously produces higher yields of technically satisfactory re-raffinates than the processes hitherto known.